Bismuth Doped TiO2 Functionalized Hetero-Junction Semiconductor Anode with an Enhanced Reactive Chlorine Generation for Use in Electrochemical Wastewater Treatment
The hetero-junction semiconductor anodes were prepared by sequential thermal decomposition method. H2IrCl6 and TaCl5 were utilized as precursors for the IrO2/Ta2O5 (Ir:Ta = 7:3) layer annealed at 525 ºC. Aqueous titanium-glycolate complex was prepared from either hydroxo-peroxo titanium or titanic acid, which was mixed with bismuth citrate solution with variable molar ratio of Ti to Bi (annealing at 425 ºC). Prepared electrodes were analyzed by SEM, XRD, FT-IR and XPS techniques. Electroanalytic experiments including cyclic voltammetry (CV) and Tafel analysis were performed to assess the OER onset-potential and current response under variable anodic potentials. The current efficiencies of reactive chlorine generation were estimated using DPD reagent under potentiostatic conditions (2~3 V versus NHE) in dilute chloride solutions (50 mM NaCl) with circum-neutral pH. In addition, the stability of the electrodes was assessed by an accelerated life test.
The Bi-doped TiO2 in the form of transparent thin film showed negligible current generation without the underlying IrO2/Ta2O5. Comparing CV data of the hetero-junction anode with IrO2/Ta2O5 anode indicated that the over-coating of Bi-doped TiO2 increases the OER onset-potential by 100~200 mV depending on the molar ratio of Ti to Bi, which was in consistent with the potentiostatic current densities reduced by 15 ~ 50%. Nevertheless, the hetero-junction fabrication was always found to increase the current efficiency of RCS generation. The highest current efficiency was observed for the Ti:Bi ratio of 7:3, which approached virtually unity (100%). A series of material characterization and electro-analysis suggested that the hydration of TiO2 provides active sites for the hydroxyl radical formation, while partial substitution with Bi resulted in a distortion in the crystalline structure and an increase in surface area. Consequently, the potentiostatic generation rate of reactive chlorine for Bi0.3Ti0.7Ox anode was almost twice of that for IrO2/Ta2O5. The accelerated life test predicted the lifetime of the hetero-junction anodes to be more than 2 year under normal operational range (300 A m-2), thereafter a dissolution of Ir was observed. Therefore, our hetero-junction fabrication can also elongate the service life without a loss of precious element.